The need for stable reference voltages is common in the design of electronic equipment. Nearly all electronic circuits require one or more sources of stable DC voltage. A variety of types of reference voltage supplies are known in the art. Reference supplies stabilized by zener diodes are often used in this application, for example. The zener diode, however, is a noisy component; further, it cannot be used with very low-voltage supplies, and it suffers from long-term stability problems. As an alternative, circuits known as "bandgap" references have become popular. Bandgap reference circuits can be operated from low-voltage sources and depend mainly upon sub-surface effects of semiconductor materials, which tend to be more stable than the surface breakdowns generally obtained with zener diodes.
A bandgap voltage reference circuit generally employs two transistors operated at different current densities, and means for developing a voltage proportional to the difference in the base-emitter voltages of those transistors (termed .DELTA.V.sub.BE). Usually, the bases of the two transistors are tied together and a resistor connects their emitters, to sense the difference in V.sub.BE 's.
A bandgap reference might more properly be called a V.sub.BE reference, as it basically involves the generation of a voltage with a positive temperature coefficient the same as the negative coefficient of a transistor base-emitter junction voltage (i.e., V.sub.BE). When the voltage with the positive temperature coefficient is added to a V.sub.BE, the resultant voltage has a zero temperature coefficient in the ideal case. Substantially all bandgap references feature the summation of a base-emitter junction voltage with a voltage generated from a pair of transistors operated with some ratio of current densities. Conventional bandgap reference circuits are explained in many texts, including P. Horowitz and W. Hill, The Art of Electronics, Cambridge University Press, Cambridge, England, 1980, at 195-199, which is hereby bandgap reference circuit is illustrated in FIG. 1.
The base-emitter voltage of a transistor exhibits a temperature-dependent function. Consequently, the output voltage of a bandgap reference circuit will exhibit a similar temperature dependency unless special steps are taken to eliminate that dependency. The thermal non-linearity of a bandgap reference cell generally is termed "curvature." Efforts have been made in the past to compensate for such curvature (as a function of temperature) to reduce long-term thermal drift. As explained in U.S. Pat. No. 4,250,445, titled "Band-Gap Voltage Reference with Curvature Correction" issued Feb. 10, 1981 to A. Paul Brokaw, the mathematical relationships regarding the variation of voltage with the temperature in bandgap devices commonly are simplified for purposes of analysis, by ignoring certain terms of the basic equation since those terms express only secondary effects. Those effects, however, can be important in some applications. Justification exists, therefore, for providing a way to minimize variations in the output voltage of a bandgap reference circuit, with respect to temperature variations.
The equations defining the output voltage dependency on temperature, for a simple three-terminal IC band-gap reference, are listed in the aforesaid U.S. Pat. No. 4,250,445, as taken from A. Paul Brokaw, "A Simple Three-Terminal IC Band-Gap Reference", IEEE J. Solid-State Circuits, Vol. SC-9, No. 6, December 1974, pp. 388-393. As stated in U.S. Pat. No. 4,250,445, the output voltage varies with temperature in such a way that an exact compensation for such variation would require quite complex circuitry, too costly for most applications.
In the circuit of U.S. Pat. No. 4,250,445, reproduced herein as FIG. 2, a degree of compensation for the second order temperature-dependency of bandgap reference output voltage is obtained by incorporating into the reference circuit, in series with the usual emitter resistor, a second resistor (R.sub.b) having a more positive temperature coefficient (TC) than the first resistor (R.sub.a, which has a nearly zero TC). The current developed in the series combination of R.sub.a and R.sub.b is proportional to absolute temperature (PTAT). The positive TC of resistor R.sub.b, together with the PTAT current flowing therethrough, produces a voltage which is partially described by a parabolic term. Under ideal conditions, the circuit elements can be so arranged that the additional voltage component resulting from the parabolic term substantially counteracts the second order variation of the voltage produced by the basic bandgap circuit. Ideal conditions do not occur in typical manufacturing environments, though. Resistor R.sub.b will generally be a diffused resistor, to attain a high, positive TC. The resistance of such a resistor is hard to control precisely and substantial variation in resistance value will occur in a manufacturing environment; moreover, such a resistance is not easily adjusted by laser trimming.
Alternatively, Palmer and Dobkin have described a circuit which provides a 12:1 reduction in output drift. The circuit, as reproduced here in FIG. 3, is relatively complicated. The temperature behavior of the collector voltage for transistor Q15 is set to be PTAT, and that of the collector current of transistor Q24 to be proportional to emitter-base voltage. This is said to create a thermal non-linearity in the difference between the base-emitter voltages of transistors Q15 and Q16 that effectively compensates for the curvature observed in the base-emitter voltages of transistors Q20 and Q22. Central to the operation of this circuit is the addition of the diode-connected transistor Q20, whose presence permits biasing of both the reference cell and its error amplifier directly from the regulated output. Apparently, only thin-film resistors are used throughout. C. R Palmer and R. C. Dobkin, "A Curvature Corrected Micropower Voltage Reference", Proceedings of the 1981 IEEE International Solid-State Circuits Conference at 58-59.
Another curvature-corrected bandgap reference circuit is described in G. C. M. Meijer et al., "A New Curvature-Corrected Bandgap Reference", IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 6, December 1982, at 1139-1143. Meijer et al. claim a 20:1 reduction in thermal non-linearity compared to conventional bandgap references. By contrast with Palmer and Dobkin, they claim to compensate directly for the non-linearity of the base-emitter voltage and to use only high-performance NPN transistors instead of lateral PNP's. Meijer et al. compensate for the non-linearity in V.sub.BE by making the collector current temperature-dependent. A schematic circuit diagram of the Meijer et al. reference is shown in FIG. 4. The four series-connected base-emitter junctions of transistors Q1-Q4 are biased at a PTAT current I.sub.PTAT, while the three series-connected base-emitter junctions of transistors Q12-Q14 are biased at a temperature-independent current, I.sub.REF. For a transistor operated at PTAT current, the thermal non-linearity in V.sub.BE about 25 percent less than that of a transistor biased at a constant current. Subtracting the three base-emitter voltages with higher non-linearity from the four with the 25 percent lower non-linearity yields a voltage V'.sub.BE which changes linearly with the temperature. The linear portion of the temperature dependence of V'.sub.BE is conventionally cancelled by connecting a series resistor R.sub.1 in the path of the PTAT current. The non-linearity of V.sub.BE (T) is somewhat dependent on the bias current, so that the compensation can be optimized by properly choosing that current.
B. S. Song and P. R. Gray have described yet another type of temperature-compensated bandgap reference which has been particularly adapted for use with CMOS technology. Their circuit employs a switched capacitor technique and does not provide continuous output, making it generally unsuitable for many cases where the present invention may be used (i.e., continuous analog environments). B. S. Song, P. R. Gray, "A Precision Curvature-Compensated CMOS Bandgap Reference," Proceedings of the 1983 IEEE International Solid-State Circuits Conference, Feb. 25, 1983, at 240-241.
From the foregoing references, it will be apparent that many prior art attempts to improve the stability and reduce the thermal non-linearity (i.e., curvature) of bandgap references have necessitated substantial increases in circuit complexity. This, of course, increases the percentage of an integrated circuit which must be devoted to reference circuits and decreases the amount of chip area available for other circuits.
Accordingly, it is an object of the present invention to provide a bandgap reference with improved compensation for its inherent temperature characteristic, with such compensation to be effective in an integrated circuit manufacturing environment.
Another object of the invention is to improve the bandgap reference of U.S. Pat. No. 4,250,445, to improve its performance under the conditions present in integrated circuit manufacturing processes.